好像复制时出了点问题。。。

from functools import reduce

str2int =  {'0': 0, '1': 1, '2': 2, '3': 3, '4': 4, '5': 5, '6': 6, '7': 7, '8': 8, '9': 9}

def str2float(s):
    if '.' not in s:
        x = s
        y = '00'                        #是否整数
    else:
        b = s.index('.')
        if b == 0:
            x = '00'
            y = s[-1:b:-1]
        else:
            x = s[:b]
            y = s[-1:b:-1]

    x1 = map(lambda s: str2int[s], x)
    f1 = reduce(lambda p,q: 10 * p + q, x1)
    y1 = map(lambda s: str2int[s], y)
    f2 = reduce(lambda m,n: m/10 + n, y1)/10
    f = f1 + f2
    return f


print(str2float('0'))
print(str2float('123.456'))
print(str2float('123.45600'))
print(str2float('0.1234'))
print(str2float('.1234'))
print(str2float('120.0034'))

def str2float(s):
    s1, s2 = s.split('.')
    return reduce(lambda x, y: x * 10 + y, map(lambda x : int(x) / (10.0 ** len(s2)), s1 + s2))